In the fourth quarter of 2013, that number was actually down to 71.8 percent — which was the lowest in the industry. The reason for that was apparently that they re-shifted some schedules to get more people flying in “the meat of the day” — 9am to 6pm — but with that came other challenges. And now get this from a context standpoint: this whole discussion is really just about nine minutes. See:

The Transportation Department counts a flight as on time if it arrives within 14 minutes of its schedule; Southwest would’ve had an 83 percent on-time rate during the 12 months ended April 30 if the DOT window were 23 minutes, Hozdulick says.

14 seems like an arbitrary number in that case, no? Anyway. 83 percent would have put them third or fourth via the chart above (depending on where you see the lines falling).